It is known that the production of tubular metal elements such as, for example, bushings substantially takes place according to four possible procedures:                cold pressing using horizontal mechanical presses;        cold or hot pressing using vertical mechanical presses and subsequent lathe machining;        turning of a previously rolled tube;        removal of shavings starting from a full blank.        
Cold pressing using multistation horizontal presses foresees the absorption of a considerable pressing power to achieve a thrust of around 1200 tons necessary to carry out the machining. These horizontal presses involve a very high investment cost to achieve a high level of productivity which can reach 40 pieces a minute.
The great speed of the machining process, consisting substantially of an extrusion operation, causes an increase in the temperature of the piece which can exceed 700 degrees. This temperature exceeds the tempering temperature of all the types of steel used to construct extrusion tools and punches with evident problems regarding the short life of these instruments. Experience shows that this problem is very evident above all in the pressing of pieces with a diameter greater than 30 mm.
Another disadvantage is represented by the fact that the tolerances relative to the longitudinal dimensions of the pieces are extremely high, at least as regards parts which require a certain precision, such as for example the bushings used for connecting rods which require tolerances of around 0.3 mm.
As far as single-station vertical presses are concerned, the production procedure for tubular elements foresees pressing in subsequent but not sequential phases, to achieve the various deformation stages of the piece.
Between one stage and the next, due to the considerable work hardening of the fibres of the rough-shaped material, it is necessary to carry out several heat treatment cycles consisting substantially of annealing.
One disadvantage is represented by the fact that this sequence of treatments has a considerable effect on the pieces as well as requiring, for each stage, lubrication of the pieces (phosphate coating) which prolongs the overall time needed to obtain the finished piece.
Another drawback is represented by the fact that, in this case too, it is extremely difficult to maintain the tolerances and the life of the tools is totally unsatisfactory again because of the high machining speed.
This processing by means of single-station vertical presses also requires final turning with a further increase in costs.
As far as hot pressing is concerned, maintaining the required tolerances and tool wear represent even more serious problems than those foreseen in the previous machining processes making this procedure uncompetitive, due also to a final stage on machine tools.
As regards rolled tube turning, this is the most commonly used procedure for bushings with an external diameter less than or equal to 50 mm. The tube, rolled by the steel works in 3-5 meter bars, is cut to size and transported to a machining centre which, by removal of the shavings, forms the external radii and the two entrances.
One drawback is represented by the fact that the raw material forming the tube costs 60-70% more than the ‘full’ steel that can be used in the other processes described above.
As regards turning starting from a blank, this process is used for bushings with an external diameter greater than 50 mm, rolled tubes of these dimensions not being available on the market.
This procedure requires the use of automatic lathes which produce pieces with a good frequency (approx. 1 every 30 seconds).
One disadvantage is represented by the fact that there is a very high rate of material wastage: around 50-55% of the original blank.
Another procedure for obtaining tubular elements consists of drawing, starting from a flat piece which is then deformed and finally finished on the machine tools.
In this case too the fibres of the material undergo considerable work hardening, altering and degrading the mechanical features of the finished product.
There is also a very high rate of production rejects, considerably increasing the costs of the finished product.
The patent U.S. Pat. No. 6,098,436 (Girardello) describes a method for the machining of metals to obtain tubes of different dimensions and for different uses, made from steel with a carbon content of between 0.10% and 0.50%, with strict tolerances.
This method foresees the use, as a starting material, of a round bar of hot rolled steel which is then cut in order to obtain at least one block which is drilled and subjected to chemical treatment. The block is then pressed and, if necessary, subjected to final turning and a heat treatment in order to obtain a finished product such as a hydraulic or oleopneumatic cylinder or a container for high pressure filters or a tube for high pressure, or a bearing, using a limited quantity of steel.